Cyclophosphamide is the generic name for 2-[bis(2-chloroethyl)amino]tetrahydro-2H-1,3,2-oxazaphosphorine-2-oxide monohydrate, a widely used antineoplastic drug chemically related to the nitrogen mustards. Cyclophosphamide was one example of a group of novel cyclic phosphoric acid ester amides which were disclosed and claimed in U.S. Pat. No. 3,018,302 granted Jan. 23, 1962 to H. Arnold, et al.
A related series of compounds bearing substituents on the oxazaphosphorine ring nitrogen was disclosed and claimed in U.S. Pat. No. 3,732,340 granted May 8, 1973 also to H. Arnold, et al.
Over the past 20 years a considerable amount of literature concerning cyclophosphamide has accumulated. Most of these references deal mainly with clinical applications of this agent as an antineoplastic drug. Cyclophosphamide has been distributed for much of that time by Mead Johnson & Company under the proprietary name CYTOXAN.RTM.. ENDOXAN.RTM. and NEOSAR.RTM. are proprietary names for similar pharmaceutical formulations of cyclophosphamide which are essentially comparable to CYTOXAN.RTM..
While cyclophosphamide comprises the monohydrate drug form, which is the easiest to isolate and with which to work, the anhydrous form also exists. No other hydrate form has been reported. As used herein, the term "cyclophosphamide" refers generically to the drug substance (i.e., either the monohydrate or the anhydrous form) the term "cyclophosphamide monohydrate" refers specifically to the monohydrate and the term "anhydrous cyclophosphamide" refers to the anhydrous form. The monohydrate form is preferred for pharmaceutical processing, as the anhydrous form readily picks up water to form the monohydrate when exposed to a relative humidity of about 20-30% or higher at about 25.degree. C. While the monohydrate is stable, nonetheless, under dry conditions (relative humidities of about 20% or less) the monohydrate begins to lose this water of hydration which can cause problems in manufacture. Because of stability limitations which may be due in part to ready interconversion between the anhydrous and monohydrate forms, it is recommended that storage temperatures for cyclophosphamide products not exceed 30.degree. C. (86.degree. F.), and preferably be stored at about 25.degree. C. (77.degree. F.).
Currently, the parenteral dosage formulations of cyclophosphamide consist of sterile packaged dry powder blend admixtures of cyclophosphamide monohydrate and sodium chloride. These premixes are dissolved in water prior to administration which can be oral as well as parenteral. It is intended that the solution itself be administered promptly after being prepared but it is satisfactory for use up to several hours after preparation. During processing and/or storage of the present dry powder premix formulation, a glassiness and/or stickiness can be acquired by the premix composition giving an unattractive material with inferior solubility characteristics and decreased potency. This deterioration is more pronounced as storage time is extended or if the upper limit of the storage temperature range is exceeded.
A common practice used with constitution of sterile solids by a suitable aqueous vehicle consists of warming the container to expedite the dissolution process, especially when the solids dissolve slowly. A study examining the effect of briefly heating cyclophosphamide solutions was reported by D. Brooke, et al. in American Journal of Hospital Pharmacy, 32:44-45 (1975). This study concluded that warming vials of cyclophosphamide in order to facilitate dissolution after adding an aqueous vehicle could decrease the potency of the final injectable product. In summary, these stability limitations and dissolution difficulties can often result in clinical usage of subpotent cyclophosphamide solutions.
Therefore, the main objective of the project culminating in the instant invention was to provide a cyclophosphamide dosage form with improved solubility characteristics and enhanced appearance, while maintaining stability comparable to the dry pre-mix composition. Unexpectedly, the unique lyophilized solid composition discovered has improved stability over the previous dry pre-mix composition.
Problems with unstable drug solutions have been handled previously by practitioners in the pharmaceutical arts by applying lyophilization, cf: L. Lachman, et al, "The Theory and Practice of Industrial Pharmacy", 2nd Ed., Lea & Febiger, Philadelphia, Pa., pp. 521-524 (1976); "Remington's Pharmaceutical Sciences", 15th Ed., Mack Publishing Co., Easton, Pa., pp. 1483-1485 (1975). The technique known as lyophilization is often employed for injectable pharmaceuticals which exhibit poor stability in aqueous solution. This process involves freeze drying, whereby ice is sublimed from frozen solutions leaving only the solid, dried components of the original liquid.
The particular advantages of lyophilization are that biologicals and pharmaceuticals which are unstable in aqueous solution yet relatively stable in the solid state can be processed and filled into dosage containers in solution, taking advantage of the relative ease of processing a liquid; dried without elevated temperatures, thereby eliminating adverse thermal effects; and then stored in the dry state in which there are relatively few stability problems. Additionally, freeze dried products are often more soluble and/or more rapidly solubilized, dispersions are stabilized, and products subject to degradation by oxidation or hydrolysis are protected.
Pharmaceuticals to be freeze dried are usually in aqueous solution ranging from 0.01 to 40% in concentration of total solids. Final moisture content of the dried product is generally below 1.0% although some products, mainly biologicals, may have a final moisture content which could range as high as about 10%. Usually, the improvement in stability of the lyophilizate, compared to the solution, is due to the absence of water in the pharmaceutical composition.
The active constituent of many pharmaceutical products, though, is present in such small quantity that if freeze-dried alone, it may not give a composition of suitable bulk and, in some cases, its presence would be hard to detect visually. Therefore, excipients are often added to increase the amount of solids present. In most applications it is desirable for the dried product cake to occupy essentially the same volume as that of the original solution. To achieve this, the total solids content of the original solution is usually about 10 to 25%. Among substances found useful for this purpose, often in combination, are sodium or potassium phosphates, citric acid, tartaric acid, gelatin, lactose and other carbohydrates such as dextrose, mannitol, and dextran; and on occasion, preservatives. Various excipients contribute appearance characteristics to the cake, such as whether dull and spongy or sparkling and crystalline, firm or friable, expanded or shrunken, and uniform or striated. Therefore, formulation of a composition to be freeze dried must include consideration not only of the nature and stability characteristics required during the liquid state, both freshly prepared and when reconstituted before use, but the characteristics desired in the final lyophilized cake. Additionally, for products to be reconstituted for parenteral usage, consideration must also be given to the pharmacological effects of excipients chosen. In some instances, there may even be chemical interaction between the active ingredient and one or more of the excipients during processing. This could, of course, result in reduced potency of the finished product.
For all the above reasons, it becomes apparent that selection of a suitable excipient or excipients for a pharmaceutical product containing a reactive, thermally labile, and inherently unstable ingredient such as cyclophosphamide is not an obvious process. Considerable testing, including drug assay, would be required, as in the instant case, for the development of such a composition. We surprisingly found that only the use of mannitol as the primary excipient gave a far superior lyophilizate compared to lyophilizates obtained using other excipients. Unexpectedly, we also discovered that a lyophilized cyclophosphamide solid composition containing about 4% moisture gave a product with superior thermal stability, compared to currently available dry powder premixes, lyophilized cyclophosphamide solid compositions with moisture levels of about 1% or less, or even cyclophosphamide itself.